In environments with challenging electromagnetic radiation sources of substantial photon energies, in particular, CT-scanners, plasma generators or EUV lithography systems, a desire exists to be able to monitor the amount of high energy photons generated by that source. In an EUV scanner the amount of contamination growth on a sensor or optical element is rather unpredictable, and implies an unpredictable decay of sensor sensitivity or mirror reflectivity, both of which in turn cause a negative effect on dose control accuracy and precision. Another desire may be to monitor and/or limit the contamination of the optical elements and the dose sensor system, and in particular, the optical systems arranged to condition the beams of radiation coming from the radiation source. Furthermore, it may be desirable to be able to monitor the amount of contamination that is accumulating in the optical path and to monitor the transmission/absorption/reflection of an optical element or set of optical elements. US2011079737A1 mentions measuring photocurrent from a EUV ML facet. The disclosed sensor devices are however sensitive to contamination on the top coating. U.S. Pat. No. 3,798,451 shows a light sensitive diode that measures a photon current. The device is unsuitable for high energy radiation because of its face plate that will not transmit the wavelengths of interest. In addition the device is sensitive to contamination. For these problems the inventions seeks to find a solution.